1. Field of the Invention
This invention relates to imaging bolometers, in particular imaging bolometers that operate without cryogenic cooling to reduce cost, size, weight and power.
2. Description of the Related Art
Imaging bolometers, particularly those that operate without cryogenic cooling, are desired in many imaging applications to reduce cost, size, weight, and power. Such devices collect photons from different points in an outside scene (as imaged on the detector by forward optics), convert these photons to heat in the individual pixels, then sense that heat to measure the image signal. In a typical configuration, each bolometer pixel comprises a micro-machined (MEMS) structure, sometimes referred to as a “bridge”, with legs that physically and thermally isolate the photo-absorptive region of the pixel from an underlying thermal sink. Absorbed photons heat the isolated portion of the pixel. In the most common configurations, this heat is sensed electrically using thermo resistance. The pixel is made from a material with a significant change in electrical resistance vs. temperature, such as Vanadium Oxide (VOX). An electrical current is run through the pixel to sense the resistance, generally running through the same legs that isolate the absorptive structure from the thermal sink. While the performance of such devices has been improving markedly over time, this structure imposes certain inherent limitations.
Unlike with photo-electric (PE) detectors, where the charge created by incoming photons is electrically integrated in a capacitor within each pixel, integration in bolometer pixels is mechanical. Absorbed photons produce heat and that heat is integrated in the thermally isolated part of the pixel. The better isolated the pixel is from the thermal sink (the larger the resistance to heat flow through the supporting legs), the better the signal to noise ratio. However, this is inevitably coupled with slower response to changing inputs (an increase in the effective time constant of the device).
To some extent, PE detectors face the same trade: the longer the integration time, the better the signal to noise ratio (within the limits of the signal integration). But, this trade is much more limiting and constrained for bolometers.
For PE detectors, integration time can be changed electrically, as shown in FIG. 1, by altering the interval over which the measured signal is integrated between some lower 100 and upper 102 limits. The trade between temporal response and sensitivity can be adjusted dynamically. Furthermore, PE detectors have a well defined finite integration time . . . with nearly no response to photons collected before the start of the integration interval, and nearly uniform gain for photons arriving at different times 104 during the interval. (Sub-frame integration techniques alter this, somewhat, but the time response is still nearly uniform when compared to a bolometer.)
In typical infra-red (IR) PE detectors, all pixels are all sampled at the same time. The integration time is adjustable, but the defined integration interval is the same shape and ends at the same time for every pixel in the array. This is referred to as a “snap-shot” readout.
Hence, the temporal response of each PE pixel can be expressed as a flat finite-impulse-response (FIR) filter. Each pixel approximates a uniform-weighted integral of the incoming flux over a programmable and finite integration time that is no larger than the frame time . . . and which may be very much smaller.
“Integration time” has no exact meaning in a bolometer. The nearest equivalent is the detector time constant. It is common, although quite incorrect, to use these terms interchangeably. The time constant of a bolometer specifies how quickly the heat produced by an absorbed photon dissipates into the thermal sink, specifically, how quickly the relative temporal response 106 falls to 1/e 108, as shown in FIG. 1, where e is Euler's number approximately equal to 2.718. Unlike with electrically integrated detectors, the response does not go to zero over a finite time.
This time constant is set by the physical structure of the bolometer, specifically by the ratio of the thermal capacity of the integrator to the thermal resistance of the supporting legs. This ratio, and hence the time constant, cannot be adjusted during use. Integration to improve SNR is purely mechanical, integrating heat within this structure, so the detector cannot adapt to changing scene conditions.
Typical bolometer pixels “integrate” continuously, so that the temporal response for each pixel is anchored to (ends at) the time at which that pixel is sampled: that is, the temporal response is measured backward from the readout time. FIG. 1 shows the response of a single pixel, sampled at time=1 frame. But, as shown in FIG. 2, typical bolometers sample pixels as they are read out during the frame interval 200, so that the temporal responses for pixels sampled near the end of the frame 202 and for pixels sampled near the front of the frame 204 are shifted with respect to reach other. This is sometimes referred to as a “rolling readout”.
The temporal response of each bolometer pixel can be expressed as an exponentially decaying infinite-impulse-response (IIR) filter. Each pixel output approximates a fixed (non programmable) exponentially weighted integral of the incoming flux over an infinite integration time, which extends into previous image frames.
Another bolometer trade has to do with the duration of the sense current used to measure the temperature of each pixel. The heat absorbed by each pixel is measured as a thermally-induced change in resistance, by applying a sense current. But, passing a current through the resistance creates unintended heat, which then competes with the sensed signal. This unintended heat must be dissipated to maintain the temperature of the thermal sink. When the pixel resistance increases, the sense current adds even more unintended heat. In typical designs where the pixel resistance increases with temperature, this creates a positive feedback. Under some circumstances this can induce thermal run-away. Hence, it is advantageous to keep the sensing-current pulse short, and to measure the temperature only once per frame, relying entirely on thermal integration for SNR gain.
Some bolometers cancel common mode errors, such as thermal reference drift, by comparing the signals between pixels that are exposed to scene radiation to pixels that are not. In some cases, the scene radiation is masked temporally, using a shutter, so that the same pixels are compared across time. In others cases, some of the pixels are permanently masked, and comparisons are made between different pixels in the same frame.
Some bolometers narrow their spectral sensitivity by placing interferometric structures in front of the absorptive element (U.S. Pat. No. 5,584,117, U.S. Pat. No. 5,629,521 and U.S. Pat. No. 7,145,143 Wood). These structures act as adjustable narrow-band filters. But, unlike conventional interference filters, the spectral pass-band may be altered dynamically by adjusting the spacing between interferometric elements. As in other bolometers, the filtered light is absorbed and the resulting heat is electrically sensed as a change in resistance. The pixel temperature accumulation and measurement mechanism is completely conventional.